The "soft" part in the cap is an early method to keep from pressurizing the MC. Just like the rubber cup that will expand to compensate for lower fluid levels in the modern reservoir. Clean it as well as you can. And yes if there is rust you have had moisture in the system, the entire system. Take it all apart. Lines, hoses, wheel cylinders and the MC. Careful inspection now will save your life later. 60 year old rubber parts? Time for some new ones. Rust and pits on a brake line? Replace. Little pits in the wheel cylinder? Hone and fit new cups. If it don't leak OK. If it does? New part. You get the idea. Enjoy
